全文获取类型
收费全文 | 10390篇 |
免费 | 184篇 |
国内免费 | 24篇 |
专业分类
化学 | 6651篇 |
晶体学 | 93篇 |
力学 | 185篇 |
数学 | 1306篇 |
物理学 | 2363篇 |
出版年
2020年 | 91篇 |
2019年 | 81篇 |
2016年 | 174篇 |
2015年 | 157篇 |
2014年 | 155篇 |
2013年 | 356篇 |
2012年 | 330篇 |
2011年 | 333篇 |
2010年 | 208篇 |
2009年 | 243篇 |
2008年 | 327篇 |
2007年 | 339篇 |
2006年 | 342篇 |
2005年 | 339篇 |
2004年 | 276篇 |
2003年 | 264篇 |
2002年 | 283篇 |
2001年 | 209篇 |
2000年 | 213篇 |
1999年 | 164篇 |
1998年 | 157篇 |
1997年 | 156篇 |
1996年 | 165篇 |
1995年 | 176篇 |
1994年 | 142篇 |
1993年 | 167篇 |
1992年 | 145篇 |
1991年 | 131篇 |
1990年 | 121篇 |
1989年 | 161篇 |
1988年 | 141篇 |
1987年 | 144篇 |
1986年 | 138篇 |
1985年 | 211篇 |
1984年 | 171篇 |
1983年 | 139篇 |
1982年 | 160篇 |
1981年 | 156篇 |
1980年 | 125篇 |
1979年 | 157篇 |
1978年 | 158篇 |
1977年 | 171篇 |
1976年 | 138篇 |
1975年 | 134篇 |
1974年 | 106篇 |
1973年 | 121篇 |
1971年 | 88篇 |
1968年 | 115篇 |
1967年 | 119篇 |
1966年 | 118篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Josef Edinger Heinz Falk Norbert Müller 《Monatshefte für Chemie / Chemical Monthly》1984,115(6-7):837-852
The diastereomeric 2,3-Dihydrobilatrienes-abc derivatives (4Z, 9Z, 15Z)-7 and (4Z, 9Z, 15E)-7 bearing a cholesterylacetate moiety in position 3 of the chromophore are prepared. The reaction sequence contains an isomerisation step providing quantitative enantioselective formation of the chiral center in position 3. Configurations and conformations of the diastereomers are elucidated using1H- and13C-NMR spectra, NOE-difference spectra, 2D-NMR experiments (NOESY) and arguments from UV-VIS and CD data. It is concluded that the hydrophobic interactions between chromophore and cholesteryl-fragment yield a compact conformation where the two substructures are in close contact with each other. On the other hand these interactions seem to be too weak to induce significant changes in the chemistry, absorption spectra and conformational characterics of the bile pigment chromophore. Therefore hydrophobic interactions between chromophor and apolar amino acid residues of the protein in phytochrome should be of minor relevance for determining spectroscopic shifts. 相似文献
62.
Burkhard Schmidt Ingo Franke Franz-Josef Witteler Michael Binder 《Helvetica chimica acta》1983,66(8):2564-2571
The novel cannabinoids (6aR, 10aR)-N-ethyl-Δ8-tetrahydrocannabinol-18-amide (15) and (6aR, 10aR, 17 RS)-N-ethyl-17-methyl-Δ8- tetrahydrocannabinol-18-amide (16) , designed as cannabinoid affinity ligands, were synthesized from the corresponding acids 11 and 12 via the N-hydroxysuccinimide esters. Amide 16 was tested in the rat and was generalized to Δ9-tetrahydrocannabinol, being 5 times less potent than the training drug. An improved synthesis of (6aR, 10aR)-17,18-didehydro-Δ8-tetrahydrocannabinol (23) is reported. As model reaction for the preparation of a tritiated Δ8-tetrahydrocannabinol, compound 23 was selectively deuterated at C(17) and C(18) in benzene/Et3N using [(C6H5)3P]3RuCl2 as catalyst. 相似文献
63.
Max Schmidt und Dieter Franz 《Fresenius' Journal of Analytical Chemistry》1965,215(5):395-400
Zusammenfassung Es wird ein Verfahren zur Bestimmung von Stibin beschrieben, das sich sowohl zur Analyse von SbH3 in organischen Lösungsmitteln als auch zur Bestimmung von flüssigem oder festem SbH3 eignet. Hierbei wird das Stibin mit überschüssigem Brom zu SbV oxydiert, danach mit SO2 zu SbIII reduziert und dieses oxydimetrisch mit Jod oder Bromat titriert.
Herrn Professor Dr. C. Mahr zum 65. Geburtstag gewidmet. 相似文献
Summary A method is proposed for the determination of stibine, applicable to its solutions in organic solvents and also to the solvent-free liquid or solid substance. The procedure involves oxidation of SbH3 with an excess of bromine to SbV, reduction to SbIII with SO2, and oxidimetric titration of the trivalent antimony with iodine or bromate.
Herrn Professor Dr. C. Mahr zum 65. Geburtstag gewidmet. 相似文献
64.
Hans-Rudolf Waespe Heinz Heimgartner Hans Schmid Hans-Jürgen Hansen Henning Paul Hanns Fischer 《Helvetica chimica acta》1978,61(1):401-429
The photochemical reactions of different allyl aryl ethers (Scheme 3) were investigated in hydrocarbons (Chap. 3.1) and in alcoholic solvents (Chap. 3.2). The composition of the photoproducts depended very much on the nature of the solvent. Irradiation (3–95 h) of different methyl substituted allyl aryl ethers ( 1, 3, 5, 7 and 11 ) with a low pressure mercury lamp (λEmiss. = 254 nm; 6 or 15 Watt) under argon (quartz vessel) resulted in the formation of 2-, 3– and 4-substituted phenols, dienones and products of consecutive reactions (Tables 1–4 and 6). The results suggested that all products were formed by homolytic cleavage of the C? O bond in the singlet state of the ethers to intermediate radical-geminates (Scheme 5) followed by radical recombination of the two fragments. No products were formed by concerted processes (Table 5, Schemes 5 and 6). Upon irradiation of allyl aryl ethers lacking alkyl substituents at position 4 ( 1 and 5 ) in protic solvents, mainly 2- and 4-allylated phenols were obtained (Tables 1 and 4); 3-allylated phenols were formed only in small amounts (0.02%). However, in aromatic hydrocarbons or cyclohexane 3-allylated phenols were obtained from 1 , 5 and 11 in significant amounts (3–11%; Tables 1, 4 and 6). E.g., upon irradiation of allyl-2,6-dimethyl-2,4-cyclohexadien-1-one ( 6 ) besides 3- and 4-allyl-2, 6-dimethyl-phenol ( 23 and 24 ). Irradiation of 5 in methanol afforded 23 and 6 only in traces, whereas 24 was the main product. 相似文献
65.
66.
Heinz Falk Thomas Schlederer Peter Wolschann 《Monatshefte für Chemie / Chemical Monthly》1981,112(2):199-207
The long standing problem of solvent dependent absorption spectra of certain bile pigments such as bilirubindimethylester has been solved by measurement of molecular masses by means of vapour pressure osmometry. Using this method and the investigation of the concentration and temperature dependence of the absorption spectra on several model compounds an association equilibrium for pigments of the rubinoid structural type could be confirmed. Moreover the optical properties of the coexisting species and the corresponding equilibrium constants could be deduced by spectroscopic techniques. Verdinoid pigments lacking free carboxylic functions on the other hand tend to be monomeric in solution. A heteroassociated species involving equimolar amounts of a verdinoid and a rubinoid pigment could be verified existing on adsorbents like silicagel.
37. Mitt.:Falk, H., Thirring, K., Tetrahedron, im Druck. 相似文献
67.
Heinz W. Gschwend 《Helvetica chimica acta》1973,56(5):1763-1775
The preparation of 2-(4-phenyl)butadienyl-piperidine 5 is described. An intramolecular Diels-Alder reaction of the intermediately formed fumaramide thereof produces stereoselectively the tricyclic lactam 6 . Its structure, as well as the configurational relationship of its 5 asymmetric centers, is corroborated on the basis of the NMR.-data. Cycloacylation of the thermodynamically stable precursors 13 and 20 leads to pentacyclic aza- or diaza-steroid type skeletons. Their structures ( 14, 16 , and 21 ) and in particular their relative configurations are elaborated. A few qualitative kinetic aspects of this intramolecular (4+2)-cycloaddition are presented. 相似文献
68.
A universal algebra isaffine complete if all functions satisfying the Substitution Property are polynomials (composed of the basic operations and the elements of the algebra). In 1962, the first author proved that a bounded distributive lattice is affine complete if and only if it does not contain a proper Boolean interval. Recently, M. Ploica generalized this result to arbitrary distributive lattices.In this paper, we introduce a class of functions on a latticeL, we call themID-polynomials, that derive from polynomials on the ideal lattice (resp., dual ideal lattice) ofL; they are isotone functions and satisfy the Substitution Property. We prove that for a distributive latticeL, all unary functions with the Substitution Property are ID-polynomials if and only ifL contains no proper Boolean interval.The research of the first author was supported by the NSERC of Canada. The research of the second author was supported by the Hungarian National Foundation for Scientific Research, under Grant No. 1903. 相似文献
69.
70.